oyster.com, and the other is the hotel room finder website Room 77. They are currently USA centric, which is a pain for people like me here in Australia. However, if you are visiting the USA, then I think they are indispensable.
Oyster is wonderful, because unlike Tripadvisor, its hotel opinions are curated, and its photo documentation of hotels and their rooms is verging on the obsessive.
Room77 comes into its own when you want to know what to expect from a rooms orientation and placement within the hotel. Using a combination of google maps, hotel room floor plans, and apparent integration with Tripadvisor, you can see what you would see from your hotel window. Rooms are given a traffic light rating (green = good, orange = OK, red = ask for a different room).
Now, as well as advising on the best room, they have recently added a booking capability (the revenue stream!).
Unlike with airline seating no currently used international hotel booking platform allows you to specify a room - so you still have to negotiate that on arrival. Last time I tried this, I kept having to refer to a couple of pages of printouts from Room 77 as I compared the room the hotel had allocated with the recommendations from the Room 77 website. Do-able, but cumbersome.
Room 77 now has a smart phone App. It looks good, but unfortunately, being in Australia, I can't download it yet, as it is only available on the iTunes USA website.
Love to hear from anyone who has had a chance to use it.